1. Field of the Invention
The present invention relates to a current balance technology, and more particularly, to a passive current balance driving apparatus.
2. Description of Related Art
Following the vigorous development in semiconductor technology, portable electronics and flat display products have become popular in recent years. Among various types of flat displays, liquid crystal displays (LCD) have become the main stream products due to its low voltage operation, no radiation, light weight and small size. In general, since an LCD panel itself does not emit light, a backlight module must be disposed below the LCD panel to provide a light source for the LCD panel.
There are generally two types of conventional backlight modules, one of which is the backlight module formed by cold cathode fluorescent lamps (CCFLs), and the other of which is the backlight module formed by light emitting diodes (LEDs). Since the LED backlight module can increase the color gamut of the LCD display, the majority of current panel vendors have replaced the CCFL backlight module with the LED backlight module.
The LED backlight module includes a plurality of LED strings arranged in parallel. Each of LED strings includes a plurality of LEDs connected in series. In practice, a DC-DC converter is generally used to convert a DC power into a DC voltage which is capable of driving the LED strings simultaneously. However, each of the LED stings may have a different load characteristics and, therefore, it can be inferred that the current flowing through each LED string is different (i.e. current unbalance), thereby resulting in a non-uniform luminance of the light source provided for the LCD panel by the LED backlight module.
In order to solve such problem, a current balance control circuit may be added into the LED backlight module to regulate the current flowing through each LED string, such that the currents flowing through the LED strings may be equal (e.g. current balance). A typical current balance control circuit may be a current mirror circuit formed by a plurality of active elements (e.g. MOSFET) or a current feedback compensation circuit. In addition, the current balance control circuit may be formed by a ready-made current regulation chip (e.g. current sink IC).
However, for semiconductor devices such as the MOSFET, the characteristic curve of the drain current (Id) and the gate-source voltage (Vgs) may be different due to the process variation. Therefore, regulating the current flowing through each LED string using the current mirror circuit formed by MOSFET or the current feedback compensation circuit can have only a limited accuracy.
In addition, the number of regulation channels of the ready-made current regulation chip is usually fixed (usually four or six regulation channels) and one regulation channel is used to regulate the current flowing through one LED string. Therefore, when the LED backlight module has, for example, ten LED strings, it must use three current regulation chips each having four regulation channels or two current regulation chips each having six regulation channels. It wastes two unused regulation channels whichever configuration is selected, thus undoubtedly creating excessive cost.